Android Phone Linux Kernel Usage and the GPL

I have an Oppo A57 phone from my australian telco and it uses "ColorOS" which is just their own cut of android.

I couldn't see the kernel source on their github, so I asked the support for it and this was the result:

Quoteoppo correspondence
Hi Martin,

Apologies once again as I have contacted our service support team, we can only provide the kernel version number for A57 as information beyond this is confidential. The kernel version number is 3.18.24-G201709301307.

Please be reminded for all the devices installed with any beta version or third-parties coding will be considered as out of warranty in Australia, as it is not customised for the usage in Australia at this moment. We advice you to install the update according to OPPO Australia website only.

Apologies once again for not able to assist you further and thank you for your patience. – Tim

Does this mean they didn't make any changes to the Kernel source and hence don't need to release it? (I may be thinking of LGPL)